A GppNHp-insensitivity factor modulates the activation of beta-adrenoceptor-coupled Gs protein in rat cortex and cerebellum

Terbutaline increases the cervical resistance of the pregnant rat in vitro

Cervical ripening is a crucial process leading to delivery. Early dilation of the pregnant cervix can contribute to premature labour. The maturity of the cervix can be characterized by its resistance to mechanical stretching. Although a number of compounds are considered to increase cervical resistance (e.g., progesterone, nitric oxide synthase inhibitors and nonsteroidal anti-inflammatory drugs), none of them seem to be safe for clinical application. Other compounds, such as beta(2)-adrenergic receptor (beta(2)-AR) agonists, have been used for several decades to stop premature myometrium contractions, but their cervical action has never been investigated. The aim of this study was to detect the effects of the beta(2)-AR agonist terbutaline on nonpregnant and late-pregnant (day 18, 20, 21 or 22) cervices isolated from Sprague-Dawley rats. Cervical resistance was measured by means of a mechanical stretching test in vitro, the beta(2)-AR density was determined by Western blot analysis, the beta(2)-AR mRNA was determined by RT-PCR, while the G-protein activation following cervical beta(2)-AR stimulation with terbutaline was evaluated via a [(35)S]GTPgammaS binding assay. Terbutaline at 10(-6) M increased the cervical resistance of the late-pregnant samples in vitro from day 18 to day 22, but did not alter the resistance of the nonpregnant samples. This cervical resistance-increasing effect was concentration dependent and antagonized with propranolol on day 21. Terbutaline was ineffective on cervical samples when gradual stretching was omitted. RT-PCR and Western blot studies revealed increased beta(2)-AR mRNA and beta(2)-AR levels respectively on day 18 of pregnancy compared with the nonpregnant cervix, but no further changes were detected up to the end of pregnancy. The [(35)S]GTPgammaS binding assay demonstrated a decreased G-protein activation on the days of pregnancy investigated, but no activation was found in the nonpregnant samples. The degree of decrease in G-protein activation by terbutaline was in harmony with its cervical resistance-increasing action. On day 21, the G-protein activation-decreasing effect of terbutaline was antagonized with propranolol. We presume that the cervical resistance-increasing effect of terbutaline is a consequence of its G-protein activation-decreasing property via beta(2)-ARs, which finally leads to an increased muscle resistance against mechanical stretching. This action of terbutaline seems unique among the smooth muscles, and may open up a new perspective in the prevention of premature labour. Clinical experience indicates that beta(2)-AR agonists will not be sufficient to stop the overall process, but their combination with more potent inhibitors of uterine contractions may be of clinical benefit.

Ontogeny of G-protein expression: control by beta-adrenoceptors

Cardiac cell homeostasis is maintained in the face of excessive beta-adrenoceptor stimulation through the process of desensitization. Desensitization is not an inherent property of these cells but rather is acquired during development; neonates given beta-agonists actually show heterologous sensitization, involving changes in the expression and catalytic activity of adenylyl cyclase (AC) as well as an increased receptor/G-protein coupling. The current study examines the role of specific G-protein components, G(s)alpha and G(i)alpha, in the ontogeny of beta-adrenoceptor responses and in the transition from agonist-induced sensitization to desensitization. Between postnatal days (PN) 6 and 15 there was a significant decrease in the 52 kDa isoform of G(s)alpha with no accompanying change of the 45 kDa form; over the same period, G(i)alpha3 also declined substantially. In contrast, the 45 kDa isoform of G(s)alpha and G(i)alpha1,2 remained fairly constant over the same period and fluoride-stimulated AC activity increased. Treatment with isoproterenol on PN2-5 did not result in any significant changes in G(s)alpha expression but robustly decreased G(i)alpha1,2. These changes were accompanied by heterologous sensitization of AC activity at the level of AC itself, evidenced by equivalent increases in the enzymatic response to fluoride and forskolin-Mn2+. Isoproterenol given to older animals (PN11-14) also caused specific loss of G(i) protein, in this case targeting G(i)alpha3, whereas G(s)alpha again was unchanged; in contrast to the younger group, the older animals displayed heterologous desensitization of AC at the level of G-protein function (specific loss of the fluoride response). These results indicate that the normal ontogenetic increase of cardiac beta-adrenoceptor coupling to AC is not dependent on the absolute amount of G-proteins, nor on the relative balance of stimulatory (G(s)) and inhibitory (G(i)) subunits. However, the ability of receptor stimulation to downregulate G(i)alpha1,2, an event which is specific to immature cardiac cells, is likely to be an important component of the resistance of the fetal/neonatal heart to agonist-induced desensitization and hypertrophy. The maintenance of cardiac beta-adrenoceptor signaling in the face of intense stimulation is likely to play an important role in the physiologic adaptations necessary to the perinatal transition.